Computed tomography is becoming more and more readily available to general practices, either as a local referral practice
or in-house in larger practices. Typically this procedure is performed only on the most complex cases and only under general
anesthesia. Recent developments have allowed faster, safer and higher quality imaging to all come together in a clinical setting.
The original computed tomographic systems were single slice sequential imaging machines. The gantry rotated about the patient
in a few seconds then the patient couch moved forward a distance dictating the slice thickness and the gantry rotated again.
These sequential systems were very slow. Often the imaging of a dog thorax required 60+ minutes for data acquisition and computer
analysis for final display of the images. The bad old days of CT.
All newer systems provide helical imaging capability. With helical (=spiral") imaging the patient moves continuously, without
pause for computer analysis, and the entire batch of data is then analyzed after the scanning is completed. This provides
faster imaging then sequential systems. However with single detector CT systems the scanning is still limited by rotation
time (up to 1 second) times the thickness of each slice imaged (preferable <1mm) times the length of the body part scanned.
So a 20cm long thorax might still require minutes to scan (i.e. 1 sec x 1 mm x 200mm = 200 seconds sec, = 3+ minutes). We
can't stop the motion under these circumstances and patients are still anesthetized with single slice helical systems.
Newer CT system are multidetector; 2, 4, 8, 16, 64, etc. With these systems we significantly decrease the time of imaging
by 1) collecting many slices of data concurrently, and 2) faster rotation time (<0.5 seconds) resulting in dramatic shorter
imaging times (0.5sec/slice x 1.0mm slice thickness x 200mm/16slices/rotation = 6 sec). Most of the imaging in dogs and cats
can be performed in 5-15 seconds with a 16 slice helical CT protocol. Scanning is best performed with sub-millimeter slice
thickness with overlap of the imaging sets. This will produce isotropic imaging.
Isotropic imaging means that the reconstructed images have the same image resolution as the original imaging plane. This makes
MPR, 3-D, and virtual endoscopy CT imaging possible.
Virtual CT imaging is possible for an array of hollow tubular organs. Because contemporary CT scanners offer isotropic, or
near isotropic, resolution, display of images does not need to be restricted to the conventional axial images. Instead, it
is possible for a software program to build a volume by 'stacking' the individual slices one on top of the other. The program
may then display the volume in an alternative manner.
Multiplanar reconstruction (MPR) is the simplest method of reconstruction. A volume is built by stacking the axial slices.
The software then cuts slices through the volume in a different plane (usually orthogonal). MPR is frequently used for examining
the spine. Axial images through the spine will only show one vertebral body at a time and cannot reliably show the intervertebral
discs. By reformatting the volume, it becomes much easier to visualize the position of one vertebral body in relation to the
Modern software allows reconstruction in non-orthogonal (oblique) planes so that the optimal plane can be chosen to display
an anatomical structure. This may be particularly useful for visualizing the structure of the bronchi as these do not lie
orthogonal to the direction of the scan.
For vascular imaging, curved-plane reconstruction can be performed. This allows bends in a vessel to be "straightened" so
that the entire length can be visualised on one image, or a short series of images. Once a vessel has been "straightened"
in this way, quantitative measurements of length and cross sectional area can be made, so that surgery or interventional treatment
can be planned.